Re: Tek TDS694C advice pls


Thank you Raymond, learned something new today.RaySent from my T-Mobile 4G LTE device------ Original message------From: Raymond Domp FrankDate: Tue, Sep 8, 2020 14:56To:;Cc: Subject:Re: [TekScopes] Tek TDS694C advice plsOn Tue, Sep 8, 2020 at 09:21 PM, Ray wrote:

On second read I checked the specs of the P6249.  So it is active (Fet) and
has higher input resistance (20 kohm) then the P6158 (1 kohm)So the P6249 will
do less loading of the measured circuit.
At higher frequencies the P6249 is a lighter load than the P6158, not because of its 20 kOhm parallel resistance vs. the P6158's 1 kOhm, but because its input capacity is significantly lower (< 1 pF) vs. the P6158's (1.5 pF).
Because of the much lower input capacitance of the P6249, its impedance decreases much slower than that of the P6058 with increasing frequency.
Be aware that at 160 MHz, the input Z of the P6249's input capacitance (if 1 pF) is around 1 kOhm, the same as the P6158's input-R. Input-R is unimportant at higher frequencies, as always with parallel capacitances. The input-Z of the P6158 at 160 MHz is about 670 Ohm, parallelled with 1 kOhm, so 400 Ohm.


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